However, the underlying molecular mechanism of HDAC inhibition remains to be fully elucidated.<h4>Objectives</h4>The study aimed to evaluate the clinical outcome of chidamide (an oral selective HDAC inhibitor for HDAC1, HDAC2, HDAC3, and HDAC10) in combination with chemotherapy in relapsed or refractory T-ALL and explore the underlying molecular mechanism of HDAC inhibition in T-ALL.<h4>Methods</h4>The clinical outcomes of 28 patients with relapsed or refractory T-ALL, who received chidamide in combination with chemotherapy were first evaluated. This evidence concerns the gene HDAC9 and acute lymphoblastic leukemia.